1. (Field of the Invention)
The present invention relates to a driving apparatus for driving an electro-acoustic transducer such as a speaker unit constituting a speaker system so that output characteristics of the transducer are improved and, more particularly, to a driving apparatus which comprises a plurality of transducer connection terminals, and can switch its driving characteristics and a transducer connection terminal by replacing a control information storage body such as a cartridge, thus preventing a wrong combination of the electro-acoustic transducer and the control information storage body.
2. (Description of the Prior Art)
As a conventional driving apparatus for driving a speaker unit assembled in a speaker system, a power amplifier whose output impedance is substantially zero is generally used. A conventional speaker system is arranged to exhibit optimal acoustic output characteristics when it is constant-voltage driven by such a power amplifier whose output impedance is substantially zero.
FIG. 6 is a sectional view of a conventional closed type speaker system. As shown in the Figure, a hole is formed in the front surface of a closed cabinet 1, and a dynamic speaker unit 3 having a diaphragm 2 is mounted in this hole.
A resonance frequency f.sub.oc of this closed type speaker system is expressed by: EQU f.sub.oc =f.sub.o (1+S.sub.c /S.sub.o)1/2 Tm (1)
A Q value Q.sub.oc of this speaker system is expressed by: EQU Q.sub.oc =Q.sub.o (1+S.sub.c /S.sub.o)1/2 Tm (2)
where f.sub.o and Q.sub.o are respectively the lowest resonance frequency and Q value of a dynamic speaker unit 3, i.e., the resonance frequency and Q value when this speaker unit 3 is attached to an infinite plane baffle. S.sub.o is the equivalent stiffness of a vibration system, and S.sub.c is an equivalent stiffness of the cabinet 1.
In the closed type speaker system, the resonance frequency f.sub.oc serves as a standard of a base sound reproduction limit of a uniform reproduction range, i.e., a lowest reproduction frequency. The Q value Q.sub.oc relates to a reproduction characteristic curve around the resonance frequency f.sub.oc. If the Q value Q.sub.oc is too large, the characteristic curve becomes too sharp around f.sub.oc. If the Q value Q.sub.oc is too small, the characteristic curve becomes too moderate. In either case, the flatness of the frequency characteristics is impaired. The Q value Q.sub.oc is normally set to be about 0.8 to 1.
FIG. 7 is a sectional view showing an arrangement of a conventional phase-inversion type (bass-reflex type) speaker system. In the speaker system shown in the Figure, a hole is formed in the front surface of a cabinet 1, and a dynamic speaker unit 3 having a diaphragm 2 is mounted in the hole. A resonance port (bass-reflex port) 8 having a sound path 7 is arranged below the speaker unit 3. The resonance port 8 and the cabinet 1 form a Helmholtz resonator. In this Helmholtz resonator, an air resonance phenomenon occurs due to an air spring in the cabinet 1 as a closed cavity and an air mass in the sound path 7. A resonance frequency f.sub.op is given by: EQU f.sub.op =c(A/lV).sup.1/2 /2.pi. TM (3)
where c is the velocity of sound, A is the sectional area of the sound path 7, l is the length of the neck of the sound path 7, and V is the volume of the cabinet 1. In a conventional bass-reflex type speaker system according to a standard setting, such a resonance frequency f.sub.op is set to be slightly lower than the lowest resonance frequency f.sub.oc.sup.' (.apprxeq.f.sub.oc) of the speaker unit 3 which is assembled in the bass-reflex type cabinet 1. At a frequency higher than the resonance frequency f.sub.op, the sound pressure from the rear surface of the diaphragm 2 inverts its phase oppositely in the sound path 7, whereby the direct radiation sound from the front surface of the diaphragm 2 and the sound from the resonance port 8 are in-phase in front of the cabinet 1, thus constituting an in-phase addition to increase the sound pressure. As a result of the inphase addition, the lowest resonance frequency of the system is lowered to the resonance frequency f.sub.op of the resonator. According to an optimally designed bassreflex type speaker system, the frequency characteristics of an output sound pressure can be expanded even to below the lowest resonance frequency f.sub.oc.sup.' and f.sub.0 of the speaker unit 3. As indicated by an alternate one long and two short dashed line in FIG. 8, a uniform reproduction range can be extended wider than those of the infinite plane baffle (indicated by a solid line) and the closed baffle (indicated by an alternate long and short dashed line).
In equations (1) and (2), the equivalent stiffness S.sub.c is inversely proportional to a volume V of the cabinet 1. Therefore, when the speaker system shown in FIG. 6 or 7 is constant-voltage driven, its frequency characteristics, in particular, low-frequency characteristics are influenced by the volume V of the cabinet 1. Thus, it is difficult to make the cabinet 1 and the speaker system compact without impairing the low-frequency characteristics.
For example, in order to compensate for bass-tone reproduction capacity decreased due to a reduction in size of the cabinet, as shown in FIGS. 9(a) to 9(d), a system of boosting a bass tone by a tone control, a graphic equalizer, a special-purpose equalizer, or the like of a driving amplifier can be employed. In this system, a sound pressure is increased by increasing an input voltage with respect to a frequency range below f.sub.oc which is difficult to reproduce. With this system, the sound pressure can be increased at frequencies below f.sub.oc. However, bad influences caused by high Q.sub.oc which is increased due to a speaker unit disposed in a compact cabinet 5, such as poor transient response at f.sub.oc caused by high Q.sub.oc, an abrupt change in phase at f.sub.oc due to high Q.sub.oc, and the like, cannot be completely eliminated. Therefore, the sound pressure of a bass tone is merely increased, and sound quality equivalent to that of a speaker system which uses a cabinet having an optimal volume V and appropriate f.sub.oc and Q.sub.oc cannot be obtained.
Furthermore, in the bass-reflex type speaker system shown in FIG. 7, if flat frequency characteristics upon constant-voltage driving are to be obtained, for example, the Q value Q.sub.oc.sup.' of the speaker unit 3 assembled in the bass-reflex cabinet is set to be Q.sub.oc.sup.' =1/.sqroot. 3, and the resonance frequency f.sub.oc.sup.' is set to be f.sub.oc.sup.' =f.sub.oc /.sqroot.2. In this manner, characteristics values (f.sub.o and Q.sub.o) of the speaker unit 3, the volume V of the cabinet 1, and dimensions (A and l) of a resonance port 8 must be matched with high precision, resulting in many design limitations. Q.sub.oc.sup.' and F.sub.oc.sup.' can be approximated by Q.sub.oc and f.sub.oc in equations (1) and (2).
FIG. 10 shows a circuit for equivalently generating a negative impedance disclosed in U.S. Pat. application No. 07/286,869 previously filed by the same assignee. According to a driver system using the circuit for generating a negative impedance (to be referred to as negative resistance driving system hereinafter) as a driving apparatus for a speaker system and causing an output impedance to include a negative resistance -R.sub.0 to eliminate or invalidate the voice coil resistance R.sub.V of a speaker, the Q.sub.oc and Q.sub.oc .sup.' can be decreased and Q.sub.op can be increased as compared to those when the speaker is constant-voltage driven by the power amplifier having an output impedance of zero. Thus, the speaker system can be rendered compact, and acoustic output characteristics can be improved.
However, an amplifier to which the negative resistance driving system of said prior application is applied has a one-to-one correspondence with a speaker system. Thus, one amplifier cannot be used for driving a plurality of types of speaker systems.
The reason for this is as follows. In the negative resistance driving method, the negative resistance value -R.sub.0 must satisfy R.sub.0 &lt;R.sub.V with respect to the voice coil resistance R.sub.V in order to avoid an oscillation caused by excessive positive feedback. Since frequency characteristics of an output sound pressure from the speaker system driven in accordance with this negative resistance value -R.sub.0 change, a change in frequency characteristics must be compensated for in addition to control of the negative resistance value -R.sub.0. However, at present, such a compensation, e.g., an equalization fitting to a kind of music to be played may be performed by a graphic equalizer or the like. However, it is relatively difficult for many users to optimally adjust even only frequency characteristics. Therefore, it is almost impossible for many users to optimally perform both control of the negative resistance value -R.sub.0 and compensation and setting of a change in frequency characteristics.
Thus, Nagi et al. proposed a driving apparatus using a circuit for generating a negative impedance, in which data of the negative resistance -R.sub.0 and frequency characteristics are stored in a control information storage body, and the storage body is set or replaced, so that optimal output characteristics of each speaker system can be easily set, and filed an application concerning this apparatus as U.S. application No. 07/353,444 assigned to the same assignee.
In such a driving apparatus, however, when there are a plurality of pairs of control information storage bodies and speaker systems, if a control information storage body to be set in the driving apparatus or a speaker system connected to this driving apparatus is erroneously selected, designed characteristics cannot be obtained. In the worst case, the negative resistance -R.sub.0 becomes too large with respect to the voice coil resistance R.sub.V of the speaker, i.e., R.sub.0 &gt;R.sub.V, and the speaker may cause oscillation. Also, if each of the control information storage bodies must have different connector specifications, system compatibility may be impaired.